It must be useful in some way to the people doing it but what could be a personal benefit?

Financial speculation likely plays a part. It might seem geopolitically insane that Europe traded cheap russian energy for expensive american/norwegian/russian but through third parties energy... but everyone who's invested in American LNG is making a killing. And Europeans who own the privatized import businesses are also making a second killing too.

That aside there just seems to be a culture of sanctions. The west is used to breaking weak nations by isolating them from western finance, capital and technology. By sheer inertia this keeps on going despite how a country like China can just keep on prospering, or how countries like Russia and India can more than make do as well. Or how all the new economic powers mean that countries like the ASS group just has options and people to talk to.

Is western finance good at insurance? Probably. But it's not like Mumbai, Shanghai or Singapore can't do the same service.

cutting off from a market suddenly is probably useful for insider trading, at least. just keep it under wraps and keep the info to yourself so you can get the juicy stocks

like one of the biggest proponents of sanctioning iran is the cashew industry, too, and iran has a very large cashew industry, so it pushes a competitor out of the market. essentially protectionism with extra steps

Send them your paper